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The present invention relates to multi-slice helical computerized tomography and more particularly to an algorithm, method and apparatus for using the same which increase the quality of resulting images by employing a new weighting algorithm.
In computerized tomography (CT) X-ray photon rays are directed through a region of interest (ROI within a patient toward a detector. Attenuated rays are detected by the detector, the amount of attenuation indicative of the make up (e.g. bone, flesh, air pocket, etc.) of the ROI through which the rays traversed. The attenuation data is then processed and back-projected according to a reconstruction algorithm to generate an image of the patient""s internal anatomy. Generally, the xe2x80x9cback projectionxe2x80x9d is performed in software but, as the name implies, is akin to physically projecting rays from many different angles within an image plane through the image plane, the values of rays passing through the same image voxels being combined in some manner to have a combined effect on the voxel in the resulting image. Hereinafter the data corresponding to rays which are back projected will be referred to as back projection rays.
During data acquisition, if a patient moves, artifacts can occur in the resulting image which often render images useless or difficult to use for diagnostics purposes. For this and other reasons, as in other imaging techniques, the CT industry is constantly trying to identify ways to reduce the duration of acquisition periods without reducing the quality of the data acquired.
In addition, because huge amounts of data are acquired during an acquisition period and the processing methods for image reconstruction from the gathered data are relatively complex, a huge number of calculations are required to process data and reconstruct an image. Because of the huge number of required calculations, the time required to process collected data and reconstruct an image is appreciable. For this reason the CT industry is also constantly searching for new processing methods and algorithms which can speed up the reconstruction process.
Various CT system features and procedures have been developed to increase data acquisition speed and to speed up the reconstruction process. Some of the more popular features and procedures including fan beam acquisition, simultaneous multiple slice acquisition, helical scanning and half-scanning. In fan beam acquisition the source is collimated into a thin fan beam which is directed at a detector on a side opposite a patient. In this manner, a complete fan beam projection data set is instantaneously generated for a beam angle defined by a central ray of the source fan beam. The source and detector are rotated about an image plane to collect data from all (e.g., typically 360 degrees) beam angles. Thereafter the collected data is used to reconstruct an image in the image plane. Thus, fan beam acquisition reduces acquisition period duration.
With respect to half-scanning, assuming a patient remains still during a data acquisition period, conjugate data acquisitions (i.e., data acquired along the same path from opposite directions) should be identical. In addition, using a fan beam, at least one ray can be directed through an image plane from every possible beam angle without having to perform a complete rotation about the patient.
For example, referring to FIG. 3, an annular gantry opening 70 is illustrated with a patient slice 42 disposed (support table not illustrated) therein and with respect to a Cartesian coordinate system where the Z-axis is into the Figure and defines a transport axis. A source 10 is illustrated in first, second, third and fourth positions as 90, 90xe2x80x2, 90xe2x80x3 and 90xe2x80x2xe2x80x3, respectively. When in the first position, source 10 generates a fan beam 40 which includes a central ray Rc and additional rays diverging therefrom along fan angles, the maximum fan angle being G. The beam angle xcex2 is defined as the angle formed by central ray Rc with respect to the vertical Y-axis.
When in the fourth position, source 10 generates a fan beam 40xe2x80x2xe2x80x3 which also includes a central ray (not illustrated) and rays diverging therefrom to form the fan beam. By rotating the source from the first to the fourth position in a clockwise direction data is collected at least once from every possible beam angle through slice 42 (i.e., the image plane). As known in the industry, data corresponding to every beam angle corresponding to a single image plane can be collected after a (xcfx80+2xcex93)2xcfx80 rotation about the patient. Because less than an entire rotation about the image plane is required to acquire the imaging data these acquisition methods and systems are generally referred to as partial-scan methods and systems and, more specifically, where data is collected during a minimal gantry rotation, the methods and systems are referred to as half-scan methods and systems. Thus, half-scan acquisition has been employed to reduce acquisition period duration in conjunction with single row detectors.
In addition, because relatively less data has to be processed in the case of half-scan imaging methods and systems to generate an image, half-scan methods and systems also have the advantage of potentially reducing data processing and reconstruction times.
As a result of the fan beam geometry of the x-ray source and the detector array, a half scan contains certain redundant data. This redundant data requires that the half scan data set be weighted with a xe2x80x9chalf scan weightingxe2x80x9d function so that the redundant data does not make a disproportionate contribution to the final image when incorporated with the non-redundant data. The weighting and reconstruction of images from a half scan data set are discussed in detail in xe2x80x9cOptimal Short Scan Convolution Reconstruction for Fanbeam CTxe2x80x9d, Dennis L. Parker, Medical Physics 9(2) March/April 1982.
While fan beams and half-scans have several advantages, often, during a diagnostics exercise a system user typically will not know the precise location within a patient of an object, cavity, etc. of interest to be imaged. For this reason, it is advantageous for a system user to be able to generate several cross sectional images in rapid succession by selecting different image/reconstruction planes. In these cases rapid data processing is extremely important to minimize delays between image generation so that a user does not lose her train of thought between image views.
Single slice detectors, fan beams and half-scans can be used to generate data in several different parallel image planes which, after data acquisition, can be used by a processor to generate an image anywhere between the image planes through interpolation/extrapolation procedures known in the art. For example, assume that during two data acquisition periods first and second data sets were acquired which correspond to first and second parallel acquisition planes, respectively, the planes separated by 0.25 inches. If a user selects an image plane for reconstructing an image which resides between the first and second acquisition planes, interpolation between data in the first and second sets can be used to estimate values of data corresponding to the selected image plane. For instance, assume that, among other rays, during the acquisition periods a first ray and a second ray were used to generate data in the first and second sets, respectively, and that the first and second rays were parallel (i.e. had the same beam and fan angles). In this case, by interpolating between the data acquired from the first and second rays generates an estimated value corresponding to a hypothetical back projection ray which is parallel to the first and second rays and which is within the image plane. By performing such interpolation to generate back projection rays for every beam and fan angle through the image plane a complete data set corresponding to the image plane is generated.
While such systems work, unfortunately, the acquisition time required to generate data corresponding to many image planes is excessive and inevitable patient movement often causes image artifacts.
One way to speed up data acquisition corresponding to several image planes is by employing a multi-row detector with a fan beam. In multi-row detector systems, a relatively thick fan beam is collimated and directed at a multi-row detector with a patient there between, each detector row in effect gathering data for a separate xe2x80x9cslicexe2x80x9d of the thick fan beam along the Z or translation axis perpendicular to a fan beam width.
After data acquisition an interface enables a system user to select an image plane from within the area corresponding to the collected data. The selected image plane is between the row centers of at least two adjacent detector rows. After image plane selection, a processor interpolates between data corresponding to adjacent rows to generate back projection rays corresponding to the selected image plane. When another image corresponding to a different image plane is desired, after selecting the plane, the processor again identifies an acquired data subset for interpolation, additional processing and back projection. Thus, multi-row detector systems further reduce data acquisition period duration where several image planes may be selected for reconstruction.
One limitation with multi-row detectors is that, during a single acquisition period, data can only be collected which corresponds to the detector thickness. To collect additional data corresponding to a greater patient volume, after one acquisition period corresponding to a first volume, the patient has to be moved along a translation axis until a second volume which is adjacent the first volume is between the source and detector. Thereafter a second acquisition process has to be performed. Similarly, to collect additional data corresponding to a third volume the patient has to be transported to another relative location with respect to the source and detector. Required translation without acquisition necessarily prolong the acquisition period and the additional acquisition time and aligning processes inevitably result in relative discomfort, additional patient movements and undesirable image artifacts.
Helical scanning systems have been developed so that data can be collected during a single acquisition period without halting patient translation during the acquisition period. In a helical scanning system, the source and detector array are mounted on opposing surfaces of an annular gantry and are rotated there around as a patient is transported at constant speed through the gantry. The X-ray beam sweeps a helical path through the patient, hence the nomenclature xe2x80x9chelical scanning systemxe2x80x9d. Data acquisition can be sped up by increasing operating pitch (i.e., table translation speed relative to gantry rotation rate).
Various combinations of the fan-beam, multi-slice, half-scan and helical scanning features have been combined to realize synergies and have been somewhat successful. For example, one system combines a multi-row fan beam detector and a fan beam source with a helical scanning procedure to rapidly acquire imaging data using a high pitch/high speed mode.
After high pitch helical data is acquired, the data is processed to generate back projection ray estimates and account for data nuances that are caused by the helical acquisition. The data processing typically includes application of a helical weighting function to the xe2x80x9cviewsxe2x80x9d (i.e., the data collected bat a specific gantry angle xcex2) collected by each detector row and then addition of the weighted views corresponding to identical gantry angles xcex2. The helical weighting functions are typically gantry angle xcex2 dependent. For instance, one exemplary helical weighting function is triangular having a value of one at a central gantry angle xcex2 that is aligned with a selected imaging plane and tapering off to zero at xcex2 angles on both sides of the central xcex2 angle corresponding to half the Z-axis distance between adjacent detector row center points. Thus, the helical weighting functions overlap. Rules are enforced such that the summation of the helical weighting function for all rows at a specific gantry angle xcex2 is one. Generally, after helical weights are applied, a weighted view from each side of a selected imaging plane are added to generate data corresponding to the imaging plane and a specific gantry angle xcex2.
Referring again to FIG. 3, in these high speed helical scanning systems, during acquisition data is acquired with source 10 at position 90, the source and detector are rotated (while data is collected) about gantry opening 70 as the patient 42 is transported there through. A processor collects data during transport and rotation from many different beam and fan angles. After source 10 rotates through a complete rotation and reaches position 90 again, additional data is gathered at that position. Because the patient 42 is transported along the Z axis during acquisition, while source 10 is at the same location 90 relative to opening 70 at the beginning and at the end of the rotation, the source and data collected are at a different Z location relative to patient 42. Hereinafter data collected for the same beam and fan angles but at different Z locations will be referred to as consecutively collected data.
To generate a slice image at a specific image plane, many interpolation techniques interpolate between consecutively collected data (i.e., data from source 10 at the same beam angle (e.g., position 90 in FIG. 3) and fan angle but at different Z (i.e., translation axis) locations). In other words, many interpolation techniques require data from more than a single source rotation to generate an image. In addition, because data from more than one rotation is required to interpolate, the data collection is relatively large and processing and reconstruction period durations are excessive. Moreover, where interpolation is between consecutively collected data, the resulting image has a xe2x80x9cthicknessxe2x80x9d characteristic which corresponds to a relatively thick patient volume which is unsuitable or at least not optimal for many diagnostic purposes.
Instead of using only interpolation to generate a slice image from a helical data set, some algorithms use data from a single half scan data set using interpolation at gantry angles where there is sufficient data to support interpolation and use extrapolation techniques at gantry angles where there is insufficient data to support interpolation. For instance, one such algorithm is described in U.S. Pat. No. 6,301,325 which issued on Oct. 9, 2001 and is entitled xe2x80x9cHalf-Scan Algorithm for use with a High Speed Multi-Row Fan Beam Helical Detectorxe2x80x9d and is commonly owned with the present invention.
Unfortunately there are several problems with extrapolation based algorithms. First, ideally only xe2x80x9cboundedxe2x80x9d data is used to generate an image where the term xe2x80x9cboundedxe2x80x9d is used to refer to data corresponding to views within a half scan gantry angle range xcex94xcex2. By limiting data used to generate an image to bounded data, generally, more accurate images that are true to the anatomical structures they purport to represent, are generated. Extrapolation is clearly an unbound process.
Second, extrapolation processes require additional views corresponding to the second and second to last detector rows (i.e., in an eight row detector the second and seventh detector rows) which complicate the computation process appreciably. To this end, extrapolation typically takes place at the ends of a half scan data range and requires helical weighting functions corresponding to the second row and the second to last row in each detector array to be extended and applied to additional views outside the normal weighting function range. For instance, in the case of an eight row detector, views collected by the seventh row are used for three purposes. First, some of the seventh row views are interpolated with sixth row views to generate interpolated views within the image plane for a sub-set of gantry angles. Second, some of the seventh row views are interpolated with eighth row views to generate interpolated views within the image plane for another subset of gantry angles. Third, additional seventh row views have to be extrapolated in conjunction with eighth row views to generate the extrapolated views required to generate an image. In addition to requiring additional calculations to apply weights to the additional row views, calculations are further complicated as special extended weighting functions have to be developed and applied by a processor.
For the reasons discussed above neither interpolation between data corresponding to more than one gantry rotation or extrapolation are optimal and therefore it would be advantageous to have an image generating algorithm that is computationally simple and accurate and that provides high quality images using a single half scan data set.
It has been recognized that different helical weighting functions can be applied to views corresponding to end detector rows in a multi-row CT detector and that a standard weighting function can be applied to each of the views corresponding to detector rows between the first and last rows (i.e., from the second to the second last row) to generate high quality images where the views used to generate the slice image correspond to a single half scan data acquisition. Importantly, using the inventive algorithm, interpolation between data from more than one gantry revolution is not necessary and extrapolation past bound data or row views can be avoided.
More specifically, where portions of helical weighting functions corresponding to adjacent detector rows overlap and therefore each helical function corresponding to an end detector row overlaps an adjacent helical function, the non-overlapping portions of each helical weighting function corresponding to each end detector row are set equal to one. After the helical weighting functions are applied to the row views, half scan weighting functions are then applied and relatively accurate images result.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.